(a) Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same, and more particularly, to a positive active material for a rechargeable lithium battery and a method of preparing the same in which the positive active material exhibits good electrochemical characteristics.
(b) Description of the Related Art
Rechargeable lithium batteries use a material from or into which lithium ions are deintercalated or intercalated as positive and negative active materials. For an electrolyte, an organic solvent or polymer is used. Rechargeable lithium batteries produce electric energy from changes in the chemical potentials of the active materials during the intercalation and deintercalation reactions of lithium ions.
For the negative active material in a rechargeable lithium battery, metallic lithium was used in the early days of development. Recently, however, carbon material, which intercalate lithium ions reversibly is used extensively instead of the metallic lithium due to problems of high reactivity toward electrolyte and dendrite formation with the metallic lithium. With the use of carbon-based active materials, the potential safety problems which are present in batteries with the metallic lithium can be prevented while achieving higher relative energy density as well as much improved cycle life. In particular, boron is added to carbonaceous materials to produce graphite coated with boron (BOC) in order to increase the capacity of the carbonaceous materials.
For the positive active material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are intercalated or deintercalated are used. Typical examples include LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2(0<X<1) or LiMnO2. Manganese-based materials such as LiMn2O4 or LiMnO2 are the easiest to prepare, are less expensive than the other materials, and have environmentally friendly characteristics. However, manganese-based materials have a low capacity. LiCoO2 is widely used as it has a good electrical conductivity and high battery voltage and is manufactured by Sony, but it is very expensive. LiNiO2 is inexpensive and has a high charge capacity, but is difficult to produce.
Rechargeable lithium batteries are classified into lithium ion batteries, lithium ion polymer batteries and lithium polymer batteries. The lithium ion battery uses a porous polypropylene/polyethylene film as a separator and a carbonate-based organic solvent dissolved with lithium ions as an electrolyte. The lithium ion polymer battery uses a porous SiO2 substrate or a polymer substrate, such as polyvinylidene fluoride into which a carbonate-based organic solvent is impregnated, as an electrolyte. Because the substrate acts as the electrolyte as well as a separator, the lithium ion polymer battery does not require an additional separator. The lithium polymer battery uses organic materials or inorganic materials such as SiO2, which have lithium ion conductivity.
With regard to the shape or formation of the rechargeable lithium battery, the different variations include a cylindrical type, a prismatic type and a coin type. The cylindrical type rechargeable lithium battery is manufactured by winding positive and a negative electrodes, and a separator into a jelly-roll to prepare an electrode element, inserting the electrode element into a battery case and adding an electrolyte to the case. The prismatic type battery is manufactured by inserting the electrode element into a prismatic case. The coin type battery is manufactured by inserting the electrode element into a coin case.
The battery may be further classified according to the type of case used. That is, there are steel or Al can batteries and pouch batteries. The can battery refers to a battery in which the case is made of a steel or Al thin sheet, and the pouch battery refers to a battery in which the case is made of a flexible material with a thickness of 1 mm or less and having a multi-layered structure such as a vinyl bag. The pouch battery has a smaller thickness and a higher degree of flexibility than the can battery.
With the miniaturization and decrease in weight of products utilizing rechargeable batteries, much research is being performed to develop batteries having good electrochemical performance such as a high capacity and long cycle life characteristics.